Light‐Induced Refractive Index Modulation in Photoactive Liquid‐Crystalline Elastomers

Abstract

A comparative study of UV‐irradiation‐induced refractive index modulation in two analogous monodomain nematic side‐chain liquid‐crystalline elastomer materials is conveyed. In one of them, mesogenic azobenzene derivatives are incorporated as pendant co‐monomers, and in the other as crosslinking units. The dependence of the optical diffraction properties on the polarization state of the probe beam reveals that diffraction is predominantly of a bulk origin for both materials. For prolonged UV exposures, the material with pendant azobenzene derivatives exhibits practically constant diffraction properties, while the material with cross‐linking azobenzene units shows a profound decrease of diffraction efficiency with increasing exposure time. The difference is attributed to photoinduced alignment of the azobenzene molecules, which is much stronger in the material with the crosslinking azobenzene units.

Mechanical Manipulation of Diffractive Properties of Optical Holographic Gratings from Liquid Crystalline Elastomers

“…As shown in our recent paper [10], the temperature dependence of the order parameter S(T) and consequently also of the extraordinary refractive index ne(T,T0) of such a film can be described by the heuristic function that is in agreement with the Landau theory of supercritical phase transitions [9,36]:…”

“…Local modifications of S result in modifications of optical birefringence of the material n b = (n e − n o ) ∝ S, where e and o denote the extraordinary and the ordinary ray, respectively. Therefore, after illumination with the periodic UV interference pattern, n o as well as n e become periodically modulated, i.e., an anisotropic optical grating structure is formed [9]. This structure decays with time due to spontaneous cis-to-trans back isomerization of the azomesogens.…”

“…As optical holographic materials, they show many intriguing properties. For instance, they exhibit recording sensitivity that is several orders of magnitude larger than conventional (non-liquid crystalline) elastomers containing similar photoisomerizable moieties [4][5][6][7][8][9]. The recording process takes place in a very nonlinear manner that allows an intricate control over the spatial structuring of the refractive index modulation.…”

Thermal characterisation of thermotropic nematic liquid-crystalline elastomers

“…[7] In their relaxed trans-state, the azobenzene molecules act as rigid rod-like molecules that contribute to the overall nematic order in the same manner as any other mesogen. Azobenzene-dyed materials have been shown to act as optically controlled actuators, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] which have been used to make micropumps and valves, [20] and show great potential in creating artificial muscles, [13][14][15][16] and are candidates for creating haptic displays. [9,17] The thermal properties of various azobenzenedyed thermotropic SCLCEs have been studied by wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and polarising optical microscopy (POM).…”

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] In recent years many SCLCEs have been synthesised with azobenzene structures as cross-linkers and/or mesogens, which has led to materials with coupled opto-mechanical properties. [6][7][8][9][10][11][12][13][22][23][24][25] These azobenzene compounds respond to light by undergoing a reversible trans→cis photo-isomerisation, where they become bent in shape, thereby lowering the nematic order and changing the mechanical response of the material. [7] In their relaxed trans-state, the azobenzene molecules act as rigid rod-like molecules that contribute to the overall nematic order in the same manner as any other mesogen.…”